High power acoustical transducer with elastic wave amplification

ABSTRACT

A high power acoustic transducer is disclosed, comprised of a number of piezoelectric elements acoustically coupled together. To maximize the available energy in each element, the piezoelectric elements are simultaneously charged by a high voltage source through a high impedance path over a relatively long period. An electrical means is provided for discharging each in succession through a low impedance path. The stored energy is thereby released by each element in a very short time to provide an acoustical pulse of high peak power. Each element is discharged with an electrical phase delay such that the elastic wave of each element adds in phase to the elastic wave of the preceding element, resulting in an amplification of the acoustic wave as it progresses down the transmission path. The entire sequence is repeated at a desired rate to produce an acoustical beam of high-peak power.

llltite States Patent 1 Miller et a1.

1 1 HIGH POWER ACOUSTICAL TRANSDUCER WITH ELASTIC WAVE AMPLlFlCATlON['15] lnventors: Darrow L. Miller, Los Angeles;

William Y. Wells, Torrance, both of Calif.

[73] Assignee: Rockwell International Corporation, El Segundo, Calif.

[22] Filed: Oct. 25, 1973 [211 Appl. No.: 409,534

151] Int. Cl H01v 7/00 Field of Search 310/81, 8.2, 8.3, 8.6.

310/98. 8; 73/675 R, 67.6, 67.8 R, 67.9; 181/.5 AG, .5 FS, .5 EM, .5 .1;318/116 1 1 Mar. 18, 1975 Clynes 310/81 Phillips 310/86 X PrimaryExaminer-Mark O. Budd Attorney, Agent. or Firm Charles T. Silberberg[57] ABSTRACT through a high impedance path over a relatively longperiod. An electrical means is provided for discharging each insuccession through a low impedance path. The stored energy is therebyreleased by each element in a very short time to provide an acousticalpulse of high peak power. Each element is discharged with an electricalphase delay such that the elastic wave of each element adds in phase tothe elastic wave of the 56 References Cited l I UNITED STATES PATENTSpreceding element, resulting in an amplification of the acoustic wave asit progresses down the transmission g 2 path. The entire sequence isrepeated at a desired rate 2. reenspan c a 3.166.731 H1965 Joy 310/81 Xto produce an dcousucll beam of hlgh peak power 3.243.648 3/1966 Yando310/81 X 13 Claims, 7 Drawing Figures l m a. l

PATENTEU 8 I975 sumanrQ HIGH POWER ACOUSTICAL TRANSDUCER WITH ELASTICWAVE AMPLIFICATION BACKGROUND OF THE INVENTION This invention relates toa high power acoustical transducer, and more particularly to anelectroacoustical transducer employing piezoelectric effects.

There is a need for high power ultrasonic transducers to detect smallanomalies deep within metallic structures and other bodies. For example,to provide adequate resolution for detecting small (3/64 inch diameter)fractures, cavities or other defects in metal structures. bonded metals,filamentary composites and the like. the sound wavelength must be smallrelative to the mean diameter of the defect, preferably to l/lO of themean diameter. That requires an ultrasonic transducer of extremely highfrequency (1 to 25MHz). Some materials exhibit high acousticalattenuation, particularly at higher frequencies. Consequently, fornondestructive inspection of critical parts, high-power high-frequencyultrasonic transducers are prerequisite to their reliable inspection.

There are other fields besides inspection of parts. For example. in themedical field X rays have been used extensively in examination of thehuman body for bone fractures, tumors, and other defects, and willcontinue to be used in those cases where a quick look will suffice. Inother cases, when the required observation would be too long for thebody to tolerate the effects of X rays, other examination techniques arerequired. Ultrasonics is a useful technology for that purpose, as wellas for therapy and dentristry. However, the body tissues absorb a greatamount of acoustic energy, particularly at high frequencies.Consequently, to find small defects, or otherwise observe small detail,ultrasonic transducers having higher transduction efficiencies at thesehigh frequencies are necessary.

An improvement in the sound echo return from a defect interface,relative to the background spatial reflection noise, can be obtained byconcentrating acoustical energy on the defect. This can be done bycollimating or focusing the beam, but the round trip absorption loss inthe material is a limiting factor. To overcome that limiting factor itis necessary to increase the energy content of the incident soundwave.

Some transducers, such as electromagnetic transducers do not permitoperation at high frequencies because of their mass. Magnetostrictiondevices are restricted to frequencies below 50 KHz because of eddycurrent losses. Some piezoelectric devices are small and are capable ofbeing operated at high frequencies up to 25 MHz, but are limited inpower. The capacitance of a piezoelectric crystal (quartz, Rochelle orlithium sulphate) or thin layer of ferroelectric material (bariumtitanate. lead titanate zirconate, lead metaniobate and the like)increases in direct proportion to the area of electrodes on oppositesides and inversely in proportion to the distance between theelectrodes. Consequently, for thin piezoelectric elements employed athigh frequency, the capacitance is very high, particularly for the moreefficient ceramic materials such as lead metaniobate or lead zirconatetitanate, which have a high dielectric constant (250 to 1,700). Highcapacitance. in turn. means low capacitive impedance. For example, at afrequency of MHz a A X /8 inch lead metaniobate piezoelectric elementwith a dielectric constant of 250 typically has a capacitive impedanceof approximately 10 ohms to a source of power applied to it. A leadzirconate titanate transducer of the same size would have an even lowerimpedance in proportion to its much higher dielectric constant.

Because of this low impedance, piezoelectric devices are very difficultto excite with a commercial 50 ohm power source. In commercial pulsingand display apparatus, the circuit usually consists of an electricalstorage capacitor (typically 330 pf) which is charged through a highresistance (typically 220 K ohms) over a relatively long period of timeand then connected across a single piezoelectric element. An electronicswitch, such as a gas thyraton or silicon controlled rec tifier (SCR) isemployed to discharge the capacitor in a short period of time. However,this has not been entirely satisfactory because not all of the energystored in the capacitor is transferred to the device; it is insteaddivided between the storage capacitor and the capacitance of thepiezoelectric device. Pulsing a plurality of piezoelectric elements insequence to increase the potential power output would require acomplicated highvoltage low-impedance pulsing system capable ofoperating at a pulsing frequency determined approximately by thevelocity of the elastic wave divided by the wavelength in thepiezoelectric material. If each unit were to be pulsed simultaneously toavoid the high frequency pulsing system, the resulting behavior (as inthe prior art) would be similar to a low-frequency device with acharacteristic frequency and behavior determined by its length and mass.

SUMMARY OF THE INVENTION An object of this invention is to producesoundwaves the magnitude of which can be selectively increased ordecreased as they progress in time through acoustically coupledpiezoelectric crystals.

Another object of this invention is to provide a high power acoustictransducer.

Ahother object is to increase the available electrical transductionenergy in an acoustically coupled piezoelectric elements as compared topulsing methods and apparatus of the prior art.

A further object is to provide a number of piezoelectric elementsacoustically coupled and to actuate some or all of the elements suchthat the elastic wave of each element actuated adds in phase with anyportion thereof of the elastic wave progressing through the elementthereby resulting in an amplification of the wave.

A further object is to provide a number of piezoelectric elementsacoustically coupled and to actuate some or all of the elements suchthat the elastic wave of each element so actuated is out of phase withan thereby subtracts from the elastic wave progressing through theelements.

Still another object is to provide a series of acoustically coupledpiezoelectric elements with means for pulsing the first element andmeans for subsequently using the previously generated elastic waveitself to trigger the remaining elements in sequence such that theelastic wave of each element coupled to the next adds in phase to theelastic wave generated in the next element.

These and other objects of the invention are achieved by a method andapparatus for charging over a relatively long period a group ofpiezoelectric elements which are polarized in a thickness mode d33parallel to the sound transmission axis. (Alternatively they may beelectrically polarized in another mode, such as a mode at a right angleto the transmission axis (c131) and arranged in series for mechanicalexpansion and contraction along the axis of sound transmission). Thepiezoelectric elements are simultaneously charged through a highimpedance means from a high voltage source. The electrical energy storedin the capacitance of the piezoelectric elements is released over arelatively short period through a low impedance means to develop a highpeak power conversion in each element discharged starting with the firstelement at one end of the group and proceeding through the group. Eachelement is discharged at a rate that permits the elastic wave created bypiezoelectric action during the extremely short discharge time of eachpreceding element to add in phase to (or subtract from if desired) theelastic wave produced by the piezoelectric element currently beingdischarged. The result is a high-power acoustic (sonic or ultrasonic)wave propagated off the end of the last element discharged.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates partially insection a first embodi ment of the invention.

FIG. 2 illustrates the complete electrical circuit for driving thetransducer of FIG. 1.

FIG. 3 illustrates a second embodiment of the present invention.

FIG. 4 illustrates still another embodiment to eliminate the requirementfor insulation between the elements.

FIGS. 5 and 6 illustrates respective charged and discharged states ofpiezoelectric elements in the embodiment of FIG. 4.

FIG. 7 illustrates a third embodiment suitable for sonic frequency pulseoperation.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, a firstembodiment of the invention is shown using only three piezoelectricelements ll, 12, and 13. In practice, any number of elements may beused. Each element is made of piezoelectric material, such as leadmetaniobate, lead titanate zirconate, barium titanate, and the like,prepared in a known way by ceramic methods and made to havepiezoelectric properties better than those of some natural crystals,such as quartz, by putting them through a cycle of polarization with ahigh electrostatic field.

Prepared and polarized piezoelectric ceramics are commercially availablein a wide range of electromechanical transduction properties withdielectric constants ranging from 5 to 1,700 and in various sizes,shapes and thicknesses (half or full wavelength). The material isusually selected with a dielectric constant, frequency constant, andelectromechanical conversion parameters to provide an optimum length orthickness (measured along the axis of sound propagation, i.e., along theaxis of desired piezoelectric mechanical strain), capacitance, andtransduction performance for a specific design frequency. For highfrequency operation, the piezoelectric material is usually made in aslab configuration for polarization in the thickness mode. (A barconfiguration is advantageously used for long wavelength elements. Thebar shaped piezoelectric elements are polarized in a right angle mode (d31) for mechanical expansion and contraction at each end). The

slabs are usually /2 wavelength thick. The exact thickness will dependon the frequency constant of the material. The following table setsforth electromechanical properties of lead metaniobate available asKezite K 81 from Keremos, Inc., Lizton, Indiana.

Table I ELECTROMECHANICAL PROPERTIES OF LEAD METANIOBATE K relativedielectric constant 250 d piezoelectric strain constant (Xl0'coulomb/newton) 35 g piezoelectric voltage constant (XI0"' voltmeter/newton) 40 d piezoelectric strain constant (Xltl coulomb/newton)-l 5 g piezoelectric voltage constant (Xl0" volt meter/newton) '7Dissipation factor at l KHz 1.0 Resistivity (ohm-cm) l0" Qm mechanical Q(thickness mode) 10 Frequency constant (thickness mode) (KC in./sec) 58Curie temperature C) 400 dh hydrostatic constant (Xl0' coulomb/newton)55 Density (gm/cm) 5.8

For an operating frequency of 2.25 MHZ, and using material having theproperties set forth in Table I, the halfwave thickness for the slabconfiguration would be about 0.025 inches and the capacitance about1,000 pf for a inch diameter slab. The piezoelectric slabs (or bars ifthe elements are polarized in a right angle mode) are arranged (stacked)along the axis of sound transmission and electrically insulated fromeach other with a low acoustical loss material as in the firstembodiment of FIG. 1. A second full-wave embodiment illustrated in FIG.4 requires no insulation between elements. Before stacking the slabs,they are prepared with electrodes in the form of thin films ofconductive material, such as vapor deposited silver, on both sides, witha small tab extending from the edge of each face through whichelectrical connections are made to a sequential trigger control circuit14.

In assembling the electroded slabs into sequentially operativepiezoelectric elements, wafers 15 and 16 made of electrical insulatingmaterial, such as mica, are cemented between the piezoelectric elements.The cement successfully used was an epoxy type EC 1469 made by MinnesotaMining and Manufacturing Corporation. However, any epoxy or other cementmay be used which can be applied in a liquid state and which sets in asolid state for form a good adhesive bond with a low interfacialacoustical loss. An epoxy is preferred for the additional insulatingqualities of that material, but other types of cement may be used. Aswill be more fully appreciated hereinafter, the material serves not onlyto hold the elements together with their electrical connecting tabs butalso provides a low reflection, low loss acoustical transmission mediabetween each element. The scope of the present invention also includesother insulating materials such as oil or liquids which may be employedin a thin film to provide a low acoustical loss between elements.

After the elements have been cemented to the insulating wafers, anacoustical back wave damper 17 is cemented to the back of the firstelement. A satisfactory acoustical damper for 2 A MHz operation consistsof the metal loaded matrix material delineated in the following table.

Table II The thickness of the damper is approximately /2 inch andcomprises the bulk of the transducer height shown in FIG. 1. It shouldtherefore be understood that the dimensions shown in that drawing arenot proportional. The dampened stack of piezoelectric elements is thenplaced in a housing open at one end with the third element flush withthe opening. To secure the elements in place with the electrical leadspassing through the housing as shown, the housing is filled with aplastic material. Alternatively, the stack of piezoelectric elements maybe potted" in plastic such that the plastic itself constitutes thehousing, or they may be placed in a housing filled with oil to provide athin insulating film between each element. In either case,quick-disconnect receptacles may be provided for connecting theelectrical leads to the outside so that the connections to the circuit14 can be easily changed. The completed threeelement high-powerultrasonic transducer is then ready to be placed on the object to beinspected. A liquid, paste or other acoustical coupling media isemployed to acoustically couple the transducer to the test article. Asuitable dry acoustical coupling matrix material is described in U.S.Pat. No. 3,663,842.

The sequential trigger control circuit for driving the transducer ofFIG. 1 is shown in FIG. 2. For ease in understanding the circuit, thepiezoelectric elements are shown separated from each other although itis understood that they are acoustically coupled by an insulatingmedium. That coupling is represented by a dotted line from one elementto the next.

A trigger pulse source transmits a single pulse for each time thetransducer is to be actuated. Between trigger pulses, the elements arecharged to a high voltage 250 V or greater from a source 21 throughseparate resistors 22, 23 and 24. These resistors are selected to belarge (typically 150 K ohms) and the charge time is controlled (byvarying a series resistor 25) to be shorter than the reciprocal of thetrigger pulse rate. This assures maximum storage of energy for releasein response to each trigger pulse.

A trigger pulse is coupled by a pulse transformer T to the gateelectrode of a silicon controlled rectifier (SCR) 26 which then fires toprovide a low impedance discharge path for the first element 11. Theelectric wave produced by the sudden change in voltage across theelement 11 causes a change in pressure across the next element 12. Forexample, when the polarization of the elements is such that the emf fromthe source 25 causes the piezoelectric material to contract in thevertical axis, the elastic wave resulting from the sudden discharge ofelement 11 will initially cause the next element [2 to be contractedeven more in the vertical axis. This produces a transient increase inthe voltage across element 12. That transient is coupled by a capacitor27 across a resistor 28 connected between the gate and cathode of an SCR29 to trigger it, thus causing the next element to discharge and therebyproduce an elastic wave or a portion thereof which adds in phase withthe elastic wave from the first element. To assure that phaserelationship, a delay element 30 may be included as shown or the RC timeconstant of the capacitor 27 and resistor 28 can be adjusted such thatthe second element does not discharge until the elastic wave from thefirst element has traveled a sufficient distance to combine in phasewith that from the second element. The third element then discharges insequence in a similar manner to complete one cycle of operation. Beforethe next trigger pulse from the source 20 occurs, all elements will berecharged in parallel for the next cycle. The result is one acousticalenergy burst for each trigger pulse, each element independentlydischarging in sequence with a traveling elastic wave initiallygenerated by the electrical discharge of the first element. This processis repeated sequentially in time at a search pulse rate of 800 to 1,000pps.

The scope of the present invention also includes having piezoelectricelements in the device which are not charged and/or discharged. Further,each piezoelectric element need not always be discharged in sequence (asto make a wider duration high energy pulse).

When required for a particular design criteria, delay element 30 or RCtime constant of capacitor 27 and resistor 28 can be adjusted such thatthe discharge of the second element combines out of phase with theelastic wave of the second element resulting in a net decrease in themagnitude of the traveling elastic wave at this position.

Transducers used in the ultrasonic frequency range have in the past beenlimited in power by their small physical size, half wave thickness andheat dissipation capabilities. Single high frequency elements have beenactuated by applying a short duration voltage pulse (about onemicrosecond) from a low impedance (50 ohm) source to the piezoelectricmaterial. This technique is not very efficient and poor power transferat ultrasonic frequencies results.

Stacked piezoelectric elements with electrical power inputs of l to 15KW have been used as transducers to generate high acoustical power butonly in the lower sonic frequency region (500 to 20 KHZ). Sometransducers consist of a group of transverse expander (45 Z cut) plateswith interleaved foil electrodes electrically connected in parallel. Theends and not the faces of the plates of the stacked crystals acttogether simultaneously to form a single lateral expansion entirelydifferent from this invention. Other combined arrangements of the priorart employ, in certain instances, thin parallel connected slabs of Y-cutlithium sulphate to form a cube or rod in which again the plates allexpand and contract in unison to provide a single strong piezoelectricvolume expansion effect which is in no way representative of theprinciple and apparatus employed in this invention. Thick slabs, longrods, large tubes, rings and other shapes are also used'in this manner,but in general, such transducers are employed only in the sonicfrequency region and when combined arrangements are employed, all thesections operate simultaneously, each individual slab, ring, or othergeometric configuration changing dimensions in unison in the samedirection and at the same time.

Another invention, U.S. Pat. No. 3,693,415, employs multiplepiezoelectric elements uniformly spaced in a row relative to theworkpiece with successive units or groups energized in a manner so thatsuccessive foci are on a path on the outer surface of the workpiece. Theangles are such that the pulses arrive substantially at the same time ata point within the workpiece. Several transducers are employed and covera substantial portion of the workpiece. This principle involves a numberof transducers each acting independently at predetermined fixed angles.The amplitude output of one is not added to the next and to the next asa function of time as in the present invention. Conventional pulsingtechniques are used. The longer acoustical path in the sample makes thispossible. Similar techniques to the above are used in the prior art forscanning a large area with surface waves using sequentially operatedmultiple transducers. Another similar invention employs a liquid withmultiple electrodes. These and other inventions of the prior art do notinvolve the principles disclosed in this invention.

The present invention is based upon simultaneously charging a number ofelements through a high impedance from a common voltage source and thendischarging the stored charges in the elements, each over a short period(less than one microsecond) through a low impedance path O.l ohm). Torecharge the elements for the next cycle, the same common highimpedanace source is used, and each element charges independently over arelatively long period. Thus, all of the piezoelectric elements aresimultaneously recharged during the interval between acoustical searchpulses from the source 20. The charge time is relatively long (0.08 to0.001 second) as compared to the envelope of the acoustical energy burst(0.2 to l microsecond). The energy stored in the elements is transformedto mechanical energy which deforms the elements making them thicker orthinner, depending upon the polarization of the elements and thepolarity of the power source. The total energy stored in each unit isproportional to the square of the battery or power supply voltage andcan be exceedingly high compared to conventional pulsing techniqueswhere only a portion of the pulsing voltage is applied across thetransducer. Since all of the stored energy can be released in about amicrosecond or less, the result is a high peak acoustical power burstfrom each element in the stack: viz., watt seconds k CB where C is thecapacitance of an element and E is the voltage of the source 21.

Assuming that 8 X 10 watt seconds of energy is stored in eachpiezoelectric element if this energy is dissipated in l microsecond. thepeak electrical power available for conversion to mechanical energywould be approximately 800 watts per element. If the energy weredissipated in about 200 nanoseconds, the peak conversion power would be4 kilowatts per element. Assuming no interface or other losses and a 100percent electromechanical conversion, the power-time behavior indictedby the energy equation would theoretically be multiplied by the numberof elements in the stack. Such an example is obviously unrealistic.Electromechanical conversion factors range between 0.5 to 0.8, and thereare interface, spatial and absorption losses. The example, however,serves to illustrate that a peak power gain of substantial magnitude canbe obtained by these techniques. If n elements are in the stack and eachis shorted by an SCR switch at a period in time substantially in phasewith the elastic wave from the preceding element, the amplitude of theelastic wave traveling down the stack is increased. The battery voltageis thus effectively multiplied n times, and an elastic wave power gainwill be obtained.

In the embodiment described with reference to FIG. 2, the sonic energyinduced voltage in each piezoelectric element is used to trigger the SCRof the next element and the receiving element. Recepticals 31 areprovided to connect suitable recording or display apparatus. Both thetransmitted signal and the echo return signal can be observed by usingthe last element 13 as a receiving as well as a transmittingpiezoelectric transducer. With adjustable delay element in the gatecircuits of the SCRs, it is possible to maximize the transmittedacoustic signal adjusting the delay elements 30 until maximum echoreturn from the defect is achieved in a standard environment.

In a second embodiment to be described with reference to FIG. 3, theSCRs are triggered in sequence through separate multivibrators. Forconvenience, all circuit components which are the same as in theembodiment of FIG. 2 are identified by the same reference numerals. Apulse from the source 20 triggers a first multivibrator 32. The leadingedge of the positive going output from the true (1) output terminal iscoupled by a differentiating circuit to the gate of the SCR 26. Thedifferentiating circuit is comprised of a capacitor 33 and a resistor 34that actuates the first element 11. When the multivibrator 32 resetsafter a predetermined period, its false (0) output goes positive andtriggers a multivibrator 35 to trigger the second element 12. After apredetermined period, the multivibrator 35 resets and triggers amultivibrator 36 to activate the third element 13. The periods of themultivibrators are set to cause the elements to be dischargedsequentially and inphase with the traveling elastic wave generated bythe first element.

Still other techniques may be devised for providing sequential triggercontrol. For example, a delay line with taps to each SCR may be used inplace of a chain of multivibrator circuits. Alternatively, digitaltechniques may be used employing a clock pulse source and a counter totime the periods between the activation of elements. Analog techniqueswould provide as an advantage a static control signal for each element,rather than a sharp triggering pulse. An advantage of a static controlsignal is that other electronic switching devices may then be used todischarge the element, such as a transistor switch which is turned onduring the presence of the control signal. For higher dischargevoltages, two transistor switches can be connected in series and turnedon simultaneously by the same control signal. Still other possibilitieswill occur to those skilled in the art. All that is essential is thatthe elements be charged over a relatively long period from a common highimpedance voltage source, and discharged in phase sequence through verylow impedance switches. The shorter the discharge period, the greaterthe peak electrical power available for conversion to mechanical energy.

Any number of elements can be used to further increase the power gainachieved upon actuating the elements in phase with the elastic wave fromthe first element. However, there is a maximum number of elements overwhich there would be no practical advantage due to the losses betweenthe elements. This occurs when the nth element contributes only about 10percent increase to the transmitted signal. Power out (P contributed bythe nth element will then be (/100) of the input power P,.

The reduction in power due to the attenuation, a, of the insulatingwafers between elements for the nth ele- For example, the practicallimit to the number of elements should not exceed 50 if a is 0.6 db.However, only a few elements may be necessary because of the energystored in each element. The gain in db for n elements over a singleelement can be derived as follows, where n 1:

power from one element P power from two elements) P (KP) power fromthree elements P (KP) (K P) power from four elements P KP K P K P powerfrom n elements P (K+K +K K") where K is the transmissibility of theinterface expressed in percent.

Other possibilities within this concept of simultaneously charging andsequentially discharging stacked piezoelectric elements will occur tothose skilled in the art in respect to how the elements are stacked forclose acoustical coupling with minimum coupling losses. The optimumwould be to stack the elements back to back without any interveninginsulating material. One arrangement, illustrated in FIG. 4, allowsadjacent elements to be cemented directly without insulators and permitsthe top and bottom surfaces of the stack to be at ground potential.

ln this arrangement, elements are discharged in pairs. The elements of apair are connected to the power supply E oppositely. All elements in thestack are arranged with the same ferroelectric polarization as indicatedby dots. Consequently, in a given pair, such as the first pair ofelements 41 and 42, charging the elements will cause the element 41 tocontract and the element 42 to expand longitudinally, i.e., along theaxis of the stack, as shown in FIG. 5. When a switch S is closed, theelement 41 will expand and the element 42 will contract simultaneously.The quiescent state of the first pair after the switch S is closed andthe stored charge has been fully discharged is illustrated in FIG. 6.The net effect is an elastic wave from the pair coupled to the next pairwhich is then actuated in phase sequence upon closing a switch S Theswitches are here represented as mechanical switches, but it isunderstood that in practice they will be implemented with electronicdevices, such as with SCRs.

Since elements operate in pairs in this alternative arrangement for thestack, it should be understood that, in the accompanying claims,piezoelectric elements" said to be charged simultaneously and dischargedin phase sequence can mean paired piezoelectric wafers, each pairconstituting an element. The principles and circuit techniques forsequential in phase discharge of elements is the same as for otherembodiments.

It is desirable to use a high charge potential as previously described.Consequently, when constructing a transducer for use in the ultrasonicregion, as the design frequency is increased flashover can occur acrossthe edges of the elements. This occurs because the half or fullwavelength thickness decreases as the frequency is increased. Theproblem can be obviated by proper selection of the piezoelectric andsurrounding materials. Piezoelectric materials are available in a widerange of dielectric constants (4.5 to 1,700).

The bottom piezoelectric element employed as a receiving transducer canbe made from a high-voltage constant (G material to provide optimumreceiving characteristics. The other elements in the stack can bemanufactured from a high-strain constant (D material to provide goodelectromechanical transduction. The receiving element may conceivably begeometrically or ultrasonically phase isolated to provide excellent nearsurface resolution. It is practical for the solidstate switchingcircuitry to be integrated with the piezoelectric stack to form acomposite transducer within one housing. This technique provides pulsingleads of minimum length, a prerequisite for ultra-high frequencyperformance. State-of-the-art microelectronic integrated circuittechniques are now developed sufficiently for this purpose.

At low (sonic) frequencies, the thickness of each element is necessarilygreater than at high frequencies due to the greater half or fullwavelength. Consequently, the capacitance of each element would be toosmall to store the sufficient energy due to the greater distance betweenelectrodes. However, the present invention can still be practiced at lowfrequencies by effectively increasing the capacitance of each element.That is accomplished by dividing the thickness of each element intosubelements, and constructing the subelements in the same manner as inthe previously described embodiments, but electrically connecting thesubelements in parallel. The paralleled subelements are thensimultaneously triggered as a single element in phase with the elasticwave traveling down the stack from the adjacent paralleled subelements.The combined length of the stacked subelements provides longerwavelengths (low frequencies) while the combined capacitance of theelectrically paralleled subelements provides greater capacity forstoring energy. FIG. 7 illustrates this technique for the embodiment ofFIGS. 1 to 3. For convenience, the same reference numerals are employedfor corresponding elements with subscripts, a, b, c for components ofsubelements. The same technique may be employed in an analogous mannerto adapt the embodiment of FIGS. 4 to 6 to low frequencies.

Although particular embodiments of the invention have been described andillustrated, it is recognized that modifications and variations mayreadily occur to those skilled in the art. Consequently, it is intendedthat the claims be interpreted to cover such modifications andvariations.

What is claimed is:

l. A method of producing sonic or ultrasonic waves from a number ofpiezoelectric elements acoustically coupled, said elements beingpolarized for expansion and contraction along the axis of soundwavetransmission comprising the steps of charging said elementssimultaneously through a high impedance from a voltage source,

discharging said elements in sequence, each element being dischargedthrough a low impedance path, and

timing the discharge of each succeeding element after the first elementhas been discharged by detecting the sonic energy induced voltageproduced in each element by the elastic wave progressing through saidelement and triggering a switch to discharge the element in which theinduced voltage is detected whereby the discharge occurs in phasesequence with the elastic wave passing through it from the precedingelement.

2. A method as defined in claim 1 wherein the timing of the discharge ofeach succeeding element after the first is predetermined to occur inphase sequence such that the elastic wave or any portion thereofproduced by each succeeding element is in phase with, and adds to, theelastic wave progressing through the group of elements.

3. A method as defined in claim 1 wherein the timing of the discharge ofeach succeeding element after the first is predetermined to occur inphase sequence that the elastic wave or any portion thereof produced byeach succeeding element is out of phase with, and subtracts from, theelastic wave progressing through said group of elements.

4. A method as defined in claim 1 wherein the timing of the discharge ineach succeeding element after the first is predetermined to occur inphase sequence such that the elastic wave or any portion thereofproduced by one or more of the succeeding elements is in phase with andadds to the elastic wave progressing through said group of elements andthe elastic wave produced by one or more of the succeeding elements isout of phase with and subtracts from the elastic wave progressingthrough said group of elements.

5. A method as defined in claim 1 wherein each element is comprised of aplurality of piezoelectric subelements acoustically coupled, and whereinsubelements of each element are discharged simultaneously, thusproviding a longer element for low frequency sonic waves of high energy.

6. Apparatus for producing an acoustical energy burst of high powerultrasonic soundwaves comprising a stack of piezoelectric elementsacoustically coupled, each element comprising a slab of piezoelectricmaterial polarized for operation in the thickness mode in a directionalong an axis of said stack, said axis being perpendicular to flat sidesof said stacked elements, and a separate conductive film on each side ofeach slab,

high impedance means for simultaneously charging the capacitance of saidelements with an electrical charge across each of said elements, and

means for discharging each element sequentially through a separate lowimpedance discharge path in phase relationship with the travelingelastic wave progressing along the sound axis, said discharging meanscomprising a separate low impedance electronic switch connected to eachelement, each switch having an anode connected to said conductive filmon one side of an element, a cathode connected to said conductive filmon the other side of said element, and a control electrode,

a capacitor between the anode and control electrode of each switchexcept the first at one end of said stack,

a resistor connected between the control electrode and cathode of eachswitch except said first one. and

means for inducing a voltage pulse between said control electrode andcathode of said first one of said switches, said voltage pulse being ofa predetermined polarity to cause said first one of said switches toconduct, thereby discharging the end one of said elements to which saidfirst one of said switches is connected, each of said elements aftersaid end one being polarized to produce an induced voltage pulse of saidpredetermined polarity in response to an elastic wave from a precedingelement in said stack.

7. The combination as defined in claim 6 wherein at least one of saidlow impedance electronic switches is a controlled rectifier.

8. Apparatus for producing soundwaves comprising:

a plurality of piezoelectric elements acoustically cou pled, each ofsaid elements being polarized for expansion and contraction along theaxis of sound transmission,

means for charging said elements simultaneously from a voltage source,

means for discharging said charged elements in sequence, each elementbeing discharged through a low impedance path, and

means for timing the discharge by detecting the sonic energy inducedvoltage produced in each element by the elastic wave progressing throughsaid element and triggering a switch to discharge the element in whichthe induced voltage is detected.

9. Apparatus for producing an acoustical energy burst of high powerultrasonic soundwaves comprising a plurality of piezoelectric elementsacoustically coupled, each element comprising a slab of piezoelectricmaterial polarized for operation in the thickness mode in a directionalong an axis of said stack, said axis being perpendicular to flat sidesof said stacked elements, and a separate conductive film on each side ofeach slab,

high impedance means for simultaneously charging the capacitance of saidelements with an electrical charge across each of said elements, and

means comprising a controlled rectifier for discharging each elementsequentially through a separate low impedance discharge path in phaserelationship with the traveling elastic wave progressing along the soundaxis, each controlled rectifier having an anode connected directly to aconductive film on one side of a unique one of said slabs, a cathodeconnected directly to a conductive film on the other side of said uniqueone of said slabs, and a control electrode connected to receive atriggering signal.

10. The combination as defined in claim 9 wherein :said means fordischarging comprises a capacitor connected between the anode andcontrol electrode of each controlled rectifier except the first at oneend,

a resistor connected between the control electrode and cathode of eachcontrolled rectifier except said first one, and

means for inducing a voltage pulse between said control electrode andcathode of said first one of said controlled rectifier, said voltagepulse being of a predetermined polarity to cause said first one of saidcontrolled rectifiers to conduct, thereby discharging the end one ofsaid elements to which said first one of said controlled rectifiers isconnected, each of said elements after said first one being polarized toproduce an induced voltage pulse of said predetermined polarity inresponse to an elastic wave from a preceding element.

ll. The combination of claim 9 wherein said means for dischargingcomprises a separate resistor between said cathode and control electrodeof each silicon controlled rectifier,

a separate capacitor having two terminals, one terminal connected tosaid control electrode of each silicon controlled rectifier, and

means for distributing a trigger pulse to second terminal of each ofsaid capacitors in sequence,

12. The combination of claim 9 wherein each of said elements includes aplurality of piezoelectric subelements, each with separate conductivefilm on each side, said subelements of an element acoustically coupledwith a polarization for operation in said thickness mode along saidaxis, all of said subelements of an element being electrically connectedin parallel for simultaneous charging, and electrically connected inparallel for simultaneous discharging by said means for discharging.

13. The combination of claim 12 wherein said means for dischargingcomprises 7 a capacitor connected between the anode and controlelectrode of each controlled rectifier except,

the first at one end,

a resistor connected between the control electrode and cathode of eachcontrolled rectifier except said first one, and

means for inducing a voltage pulse between said control electrode andcathode of said first one of said controlled rectifier, said voltagepulse being of a predetermined polarity to cause said first one of saidcontrolled rectifiers to conduct, thereby discharging the end one ofsaid elements to which said first one of said controlled rectifiers isconnected, each of said elements after said first one being polarized toproduce an induced voltage pulse of said predetermined polarity inresponse to an elastic wave from a preceding element.

l l= l=

1. A method of producing sonic or ultrasonic waves from a number ofpiezoelectric elemEnts acoustically coupled, said elements beingpolarized for expansion and contraction along the axis of soundwavetransmission comprising the steps of charging said elementssimultaneously through a high impedance from a voltage source,discharging said elements in sequence, each element being dischargedthrough a low impedance path, and timing the discharge of eachsucceeding element after the first element has been discharged bydetecting the sonic energy induced voltage produced in each element bythe elastic wave progressing through said element and triggering aswitch to discharge the element in which the induced voltage is detectedwhereby the discharge occurs in phase sequence with the elastic wavepassing through it from the preceding element.
 2. A method as defined inclaim 1 wherein the timing of the discharge of each succeeding elementafter the first is predetermined to occur in phase sequence such thatthe elastic wave or any portion thereof produced by each succeedingelement is in phase with, and adds to, the elastic wave progressingthrough the group of elements.
 3. A method as defined in claim 1 whereinthe timing of the discharge of each succeeding element after the firstis predetermined to occur in phase sequence that the elastic wave or anyportion thereof produced by each succeeding element is out of phasewith, and subtracts from, the elastic wave progressing through saidgroup of elements.
 4. A method as defined in claim 1 wherein the timingof the discharge in each succeeding element after the first ispredetermined to occur in phase sequence such that the elastic wave orany portion thereof produced by one or more of the succeeding elementsis in phase with and adds to the elastic wave progressing through saidgroup of elements and the elastic wave produced by one or more of thesucceeding elements is out of phase with and subtracts from the elasticwave progressing through said group of elements.
 5. A method as definedin claim 1 wherein each element is comprised of a plurality ofpiezoelectric subelements acoustically coupled, and wherein subelementsof each element are discharged simultaneously, thus providing a longerelement for low frequency sonic waves of high energy.
 6. Apparatus forproducing an acoustical energy burst of high power ultrasonic soundwavescomprising a stack of piezoelectric elements acoustically coupled, eachelement comprising a slab of piezoelectric material polarized foroperation in the thickness mode in a direction along an axis of saidstack, said axis being perpendicular to flat sides of said stackedelements, and a separate conductive film on each side of each slab, highimpedance means for simultaneously charging the capacitance of saidelements with an electrical charge across each of said elements, andmeans for discharging each element sequentially through a separate lowimpedance discharge path in phase relationship with the travelingelastic wave progressing along the sound axis, said discharging meanscomprising a separate low impedance electronic switch connected to eachelement, each switch having an anode connected to said conductive filmon one side of an element, a cathode connected to said conductive filmon the other side of said element, and a control electrode, a capacitorbetween the anode and control electrode of each switch except the firstat one end of said stack, a resistor connected between the controlelectrode and cathode of each switch except said first one, and meansfor inducing a voltage pulse between said control electrode and cathodeof said first one of said switches, said voltage pulse being of apredetermined polarity to cause said first one of said switches toconduct, thereby discharging the end one of said elements to which saidfirst one of said switches is connected, each of said elements aftersaid end one being polarized to produce an induced voltage pulse of saidpredetermined polarity in response to an Elastic wave from a precedingelement in said stack.
 7. The combination as defined in claim 6 whereinat least one of said low impedance electronic switches is a controlledrectifier.
 8. Apparatus for producing soundwaves comprising: a pluralityof piezoelectric elements acoustically coupled, each of said elementsbeing polarized for expansion and contraction along the axis of soundtransmission, means for charging said elements simultaneously from avoltage source, means for discharging said charged elements in sequence,each element being discharged through a low impedance path, and meansfor timing the discharge by detecting the sonic energy induced voltageproduced in each element by the elastic wave progressing through saidelement and triggering a switch to discharge the element in which theinduced voltage is detected.
 9. Apparatus for producing an acousticalenergy burst of high power ultrasonic soundwaves comprising a pluralityof piezoelectric elements acoustically coupled, each element comprisinga slab of piezoelectric material polarized for operation in thethickness mode in a direction along an axis of said stack, said axisbeing perpendicular to flat sides of said stacked elements, and aseparate conductive film on each side of each slab, high impedance meansfor simultaneously charging the capacitance of said elements with anelectrical charge across each of said elements, and means comprising acontrolled rectifier for discharging each element sequentially through aseparate low impedance discharge path in phase relationship with thetraveling elastic wave progressing along the sound axis, each controlledrectifier having an anode connected directly to a conductive film on oneside of a unique one of said slabs, a cathode connected directly to aconductive film on the other side of said unique one of said slabs, anda control electrode connected to receive a triggering signal.
 10. Thecombination as defined in claim 9 wherein said means for dischargingcomprises a capacitor connected between the anode and control electrodeof each controlled rectifier except the first at one end, a resistorconnected between the control electrode and cathode of each controlledrectifier except said first one, and means for inducing a voltage pulsebetween said control electrode and cathode of said first one of saidcontrolled rectifier, said voltage pulse being of a predeterminedpolarity to cause said first one of said controlled rectifiers toconduct, thereby discharging the end one of said elements to which saidfirst one of said controlled rectifiers is connected, each of saidelements after said first one being polarized to produce an inducedvoltage pulse of said predetermined polarity in response to an elasticwave from a preceding element.
 11. The combination of claim 9 whereinsaid means for discharging comprises a separate resistor between saidcathode and control electrode of each silicon controlled rectifier, aseparate capacitor having two terminals, one terminal connected to saidcontrol electrode of each silicon controlled rectifier, and means fordistributing a trigger pulse to second terminal of each of saidcapacitors in sequence.
 12. The combination of claim 9 wherein each ofsaid elements includes a plurality of piezoelectric subelements, eachwith separate conductive film on each side, said subelements of anelement acoustically coupled with a polarization for operation in saidthickness mode along said axis, all of said subelements of an elementbeing electrically connected in parallel for simultaneous charging, andelectrically connected in parallel for simultaneous discharging by saidmeans for discharging.
 13. The combination of claim 12 wherein saidmeans for discharging comprises a capacitor connected between the anodeand control electrode of each controlled rectifier except the first atone end, a resistor connected between the conTrol electrode and cathodeof each controlled rectifier except said first one, and means forinducing a voltage pulse between said control electrode and cathode ofsaid first one of said controlled rectifier, said voltage pulse being ofa predetermined polarity to cause said first one of said controlledrectifiers to conduct, thereby discharging the end one of said elementsto which said first one of said controlled rectifiers is connected, eachof said elements after said first one being polarized to produce aninduced voltage pulse of said predetermined polarity in response to anelastic wave from a preceding element.